1. Field of the Invention
The present invention relates to a sheet-transporting device provided in various sheet-employing apparatuses, such as image-forming apparatuses and other office equipment. In such image-forming apparatuses, an appropriate transfer-type image-forming process (such as an electrophotographic-type image-forming process) or a direct-type image-forming process is used. Examples of the image-forming apparatus include printers, facsimile machines, and copying machines.
2. Description of the Related Art
FIG. 7 is a schematic structural view of a conventional image-forming apparatus comprising a sheet-feeding section and a register section which temporarily stops the leading edge of a recording sheet fed from the sheet-feeding section, forms a loop in the recording sheet to correct any oblique movement of the recording sheet, and sends the recording sheet to an image-forming section at a predetermined timing. The conventional image-forming apparatus roughly comprises an image-input section 90R and an image-output section 90P.
The image-input section 90R is, for example, an image information photo-reader mechanism. Image information to be recorded is input to the image-input section 90R, and variously processed. Then, an image signal carrying the image information is sent to the image-output section 90P. The image-input section 90R may be an optical device for exposing an image-carrying member of an image-forming section 91 of the image-output section 90P with light carrying an image of an original by projecting and focusing the light thereon.
The image-output section 90P is a transfer-type image-recording mechanism using a transfer drum 93. It comprises the image-forming section 91; first to third sheet-feeding units 92A to 92C, serving as sheet-feeding sections; a sheet-transporting section 97 disposed in front of a register roller pair 96 serving as a register section; a transfer drum 93; a fixing unit 94; and a control unit 95. The image-forming section 91 is used to form a transferable image, such as a toner image, on an image-carrying member, such as a photoelectric member, a dielectric member, or a magnetic member, by a suitable image-forming process, such as the electrophotographic-type image-forming process, an electrostatic-recording process, or a magnetic-recording process, in accordance with the image information input to the image-output section 90P. The sheet-transporting section 97 transports a recording sheet P fed from any one of the sheet-feeding units 92A to 92C to the register roller pair 96. The transfer drum 93 holds the recording sheet P transported from the register roller pair 96 and transports it to a transfer section T of the image-forming section 91. The fixing unit 94 fixes the image to the recording sheet P onto which the image has been transfer at the transfer section T of the image-forming section 91. The control unit 95 controls all of the operations of the image-forming apparatus.
The first and the second sheet-feeding units 92A and 92B are incorporated in the image-output section 90P as top and bottom mechanisms for automatically separating sheets one by one for feeding. Starting from the topmost recording sheet, the recording sheets P loaded in and held by a sheet-feed cassette are fed one sheet at a time as a result of cooperation of a corresponding sheet-feed roller and a corresponding separating member based on a sheet-feed start signal. The third sheet-feeding unit 92C is a manual sheet-feeding device. When it is used, a recording sheet P is inserted into the image-forming apparatus from a manual sheet-feed table, and is drawn into the image-forming apparatus in order to be fed to transfer section T.
When the image-forming apparatus starts to operate, a recording sheet P is fed from any one of the first to third sheet-feeding units 92A to 92C, and transported to the register roller pair 96 through the transporting section 97 by transporting roller pairs 98A, 98B, and 98C, and a curling roller set 99. The transporting roller pairs 98A to 98C and the curling roller set 99 can rotate in two different of rotational modes, or at two transporting speeds. In one mode, they rotate at ordinary transporting speeds. In the other mode, they rotate at speeds greater than the ordinary transporting speeds.
At the moment the edge of the recording sheet P fed from any one of the sheet-feeding units 92A to 92C is nipped by the corresponding first transporting roller pair (the transporting roller pair 98A if the sheet has been fed from the first sheet-feeding unit 92A; the transporting roller pair 98B if the sheet has been fed from the second sheet-feeding unit 92B; and the curling roller set 99 if the sheet has been fed from the third sheet-feeding unit 92C) rotating at the ordinary transporting speed, the ordinary transporting speeds of the pairs of transporting rollers 98A and 98B and the pair of curling rollers 99 are switched to the higher transporting speeds in order to transport the sheet to the register roller pair 96 at a high speed. By this speed-controlling operation, the time from the start of the sheet-feeding operation to the start of the image-forming operation can be reduced compared to that when the recording sheet is transported only at the ordinary transporting speed. The rotational speed of each roller is changed by switching the rotational speed of a corresponding motor (not shown), such as a stepping motor. Each motor is directly connected to its corresponding roller. For example, when the transporting speed is to be quadrupled from 150 mm/s to 600 mm/s, the number of rotations of the driving motor of each roller is increased by a factor of 4. In order to reduce loss of transporting speed, the speed switching is carried out in a short time period, that is, a time period of the order of 100 ms.
FIG. 4 illustrates the curling roller set 99 in detail. The pair 99 comprises two upper rollers 99A and 99B and one lower roller 99C. The two upper rollers 99A and 99B are pressed against the lower roller 99C by a spring or other biasing means (not shown). As shown in FIG. 4, when the recording sheet P passes between the two upper rollers 99A and 99B and the lower roller 99C, the recording sheet P is curled upward into a convex shape. When the recording sheet P is curled, the recording sheet P is more easily attracted to the transfer drum 93.
However, since the one lower roller 99C is pressed by the two upper rollers 99A and 99B, the driving torque of the curling roller set 99 is larger than those of the transporting roller pairs 98A and 98B. For example, while the driving torques of these transporting roller pairs 98A and 98B are of the order of 1 kgfxc2x7cm, the driving torque of the curling roller set 99 is of the order of 2.5 kgfxc2x7cm.
An image corresponding to relevant image information is formed on the image-carrying member in the image-forming section 91. In synchronism with an image-recording start signal, the register roller pair 96 starts to rotate, and the transporting section 97 returns to its recording-sheet-transporting state. The recording sheet P is transported from the register roller pair 96 to the rotating transfer drum 93, and held by the outer face of the rotating transfer drum 93 in order to be transported to the transfer section T. This causes the portions of the image formed on the image-carrying member of the image-forming section 91 to be successively transferred onto the recording sheet P.
There are several methods by which the recording sheet P can be held by the outer face of the transfer drum 93. The methods include electrostatic suction and air suction, towards a transporting medium, such as a thin dielectric film.
The timing at which the recording sheet P is transported by the register roller pair 96 is controlled such that when an edge of the image carried by the image-carrying member of the image-forming section 91 reaches the transfer section T, an edge of the recording sheet P is exactly at the transfer section T, whereby they are aligned (or subjected to registration).
The recording sheet P to which the image has been transferred at the transfer section T is separated from the outer face of the rotating transfer drum 93 and is transported to the fixing unit 94. The image on the recording sheet P is fixed thereto by heat and/or pressure, after which the recording sheet P passes through a sheet-discharge section and is discharged to a sheet-discharge tray disposed outside the image-forming apparatus. At the fixing unit 94, oil is applied to the recording sheet P in order to prevent it from being wound around the fixing rollers.
However, the conventional image-forming apparatus in which the rotational speeds of the sheet-transporting systems are controlled so that the rotational speeds are switched between two speeds has the following problems.
Since the rotational speeds of the transporting roller pairs 98A and 98B, and the curling roller set 99 are switched from low speeds to high speeds in about 100 ms, a very short time, large torques due to acceleration are produced during this short time. This may cause the rollers 99A and 99B, and the curling rollers 99 to rotate improperly due to large motor loads, or may cause sheet jamming resulting from the improper rotation. Since the inertial moments of the roller pairs 98A, 98B, and the curling roller set 99 are of the order of 2 kgxc2x7cm2, acceleration torques in the order of 1.2 kgfxc2x7cm are produced when an attempt is made to quadruple the transporting speeds from 150 mm/s to 600 mm/s in the switching time of the order of 100 ms.
As mentioned above, the driving torques of the transporting roller pairs 98A and 98B are in the order of 1 kgfxc2x7cm, and of the curling roller set 99 is in the order of 2.5 kgfxc2x7cm. At the moment the speeds are switched, the aforementioned acceleration torques are added to the torque values corresponding thereto, so that the required driving torques of the transporting roller pairs 98A and 98B are 2.2 kgfxc2x7cm, while the required driving torque of the roller set 99 is 3.7 kgfxc2x7cm. Therefore, the driving motor used to rotationally drive the curling roller set 99 needs to be larger and to have a larger output than the driving motors used to rotationally drive the transporting roller pairs 98A and 98B.
However, the high-output, large motor used to rotationally drive the curling roller set 99 is more costly than the motors used to rotationally drive the transporting roller pairs 98A and 99B. In addition, this high-output, large motor is used at only one location, so that failures tend to occur when purchasing parts or during assembly. Further, it takes up a lot of space, thereby increasing the size of the image-forming apparatus.
One possible way to overcome the above-described problems is by replacing each member rotationally supporting each curling roller with a driving-load-reducing member, such as a bearing. This reduces the curling roller loads to a level equal to those on the transporting rollers.
However, since bearings are expensive, mounting driving-load-reducing members results in an increase in total costs. As shown in FIG. 4, the two upper rollers 99A and 99B are relatively small-diameter rollers and are spatially disposed close together. Therefore, when bearings or the like are inserted into the structure of the curling roller set 99, the structure becomes complicated.
Another way to overcome the above-described problems is by prolonging the time of switching the rotational speeds of the transporting roller pairs 98A and 98B, and the curling roller set 99 to, for example, a time in the order of 500 ms, as shown in FIG. 5. There is a discussion related to this method in, for example, Japanese Patent Laid-Open Publication No. 6-312852. When this method is used, the acceleration torques are reduced to levels in of the order of 0.2 kgfxc2x7cm, so that the driving torques of the transporting roller pairs 98A and 98B are reduced to values of the order of 1.2 kgfxc2x7cm, and the driving torque of the curling roller set 99 is reduced to a value of the order of 2.7 kgfxc2x7cm.
However, in this case, as shown in FIG. 5, the times required for accelerating the transporting roller pairs 98A and 98B and the curling roller set 99, are longer, so that the total amount that a recording sheet P is transported is reduced in correspondence with the delay times. Compared to the case where the transporting speeds are switched in 0.1 sec., the transportation of the recording sheet P is delayed by slightly less than 0.5 sec., so that it takes extra time to form an image, thus increasing the total image-forming time. In addition, during the switching between transporting speeds, the speeds actually become unstable as indicated by the solid lines in FIG. 8, so that prolonging the speed-switching time increases variables, such as the location to which the recording sheet P is transported. Therefore, failures caused by an unstable total controlling operation or sheet jamming caused by variations in the recording-sheet P location may result.
Accordingly, it is an object of the present invention to make it possible to eliminate the problem of size increases of an image-forming apparatus resulting from the use of a large motor. It is another object of the present invention to make it possible to eliminate the problem of increases in cost resulting from the addition of, for example, a bearing. It is still another object of the present invention to make it possible to eliminate the problem of lengthening of the total image-forming time caused by prolonging the time of switching between recording-sheet transporting speeds.
To this end, according to one aspect of the present invention, there is provided a sheet-transporting device comprising:
first transporting means for transporting sheets; and
second transporting means for transporting the sheets at a location downstream from the first transporting means in a sheet-transporting direction, a driving torque of the second transporting means being greater than a driving torque of the first transporting means;
wherein a speed of the first transporting means and a speed of the second transporting means are each switchable between a first transporting speed and a second transporting speed which is greater than the first transporting speed; and
wherein a time required to switch the speed of the second transporting means from the first transporting speed to the second transporting speed is longer than a time required to switch the speed of the first transporting means from the first transporting speed to the second transporting speed.
According to another aspect of the present invention, there is provided a sheet-transporting device comprising:
sheet-feeding means for feeding sheets;
first transporting means disposed downstream from the sheet-feeding means in a sheet-transporting direction; and
second transporting means disposed downstream from the first transporting means in the sheet-transporting direction, with a driving torque of the second transporting means being greater than a driving torque of the first transporting means;
wherein when the sheets are being fed at a first transporting speed by the sheet-feeding means, the first transporting means and the second transporting means also rotate at the first transporting speed, and after a predetermined time from completion of sheet feeding, speed switching of the first transporting means and speed switching of the second transporting means from the first transporting speed to a second transporting speed that is greater than the first transporting means are started, with a time required to switch the speed of the second transporting means from the first transporting speed to the second transporting speed being longer than a time required to switch the speed of the first transporting means from the first transporting speed to the second transporting speed.
In accordance with yet another aspect of the present invention there is provided image forming apparatus which includes the above sheet transporting devices together with means for forming an image on the sheets transported thereby.